本文透過計算流體力學軟體(CFD-RC)模擬阻礙物後方的魚隻擺動，分析阻礙物所產生的渦旋與魚身擺動相位的關係。本文於不同距離下(d=0.25L,0.5L,1L,3L,d:圓柱與魚身前緣的距離;L:魚身長度)讓魚在一流速0.45 m/s之自由流中游動，並使圓柱渦旋流逸的頻率和魚擺動頻率同步，再改變魚身擺動的相位，探討阻力功率和游動功率的變化。研究結果顯示在不同距離中，d=0.25L的渦旋流逸受到魚的抑制；在0.5L和1L之位置，阻力功率相較於游動功率之變化更為明顯，此時若分別改變魚身擺動相位為260度至360度和180度至280度時，其平均阻力功率會為負值，表示魚隻此時可從外界流場獲得一向前的推進力。在d=0.5L且擺動相位為260度至300度的平均游動功率也為負值，表示魚隻可不耗損自己擺動能量，便可達到推進的效果。d=0.5L、1L和3L的平均游動功率最小值，其擺動相位分別為280度、180度和136度，皆為魚身擺動相位和高壓移動區同相時，亦即魚頭下擺時，上方會有一高壓區向下推，擺動消耗就會較少；而觀其平均阻力功率最小值的擺動相位，則分別為300度、260度和238度，皆在魚身擺動相位和圓柱渦旋到達的位置同步時，亦即魚頭擺動向下，渦旋的位置應於魚頭上方；魚頭擺動向上，渦旋位置應在魚頭下方，魚尾部分也是如此，亦即魚身部份與渦旋位置最好要是在上下相反的兩側，以減少阻力的產生。因此魚如果想於湍急的水流中游動，並距離阻礙物不遠處，尋求最佳的游動推進時，它必須跟隨著前方的渦旋上下擺動，以達到最少的能量消耗。This research was to study the characteristics of the flow field with an undulating fish behind a circular cylinder. The effect of the phase of undulating fish and the shedding vortices from the cylinder upon the swimming power required was computed through CFD software (CFD-RC).Under different distances (d=0.25L, 0.5L, 1L, 3L, where d is the distance between cylinder and the leading edge of fish, L is the body length), a free stream with flow speed of 0.45 m/s passed through the fish body, after synchronized the shedding frequency of cylinder and the undulating frequency of fish, effects of the phase angle upon the drag power and swimming power were studied.Results demonstrated that the required drag power was higher than the swimming power when the distance was shorter than 1L. The smallest drag power consumed was observed when the vortices induced by the cylinder matched the phase of undulating fish.To conclude, for a fish to swim in a high speed flow behind an obstacle, it must adjust the phase according to the upstream shedding vortices to save the swimming energy.